This invention relates generally to compositions and methods useful in the treatment of certain infectious diseases. More specifically, the invention relates to compositions which inhibit proteases, such as malaria cysteine protease. Compounds that inhibit these proteases are useful in the prevention and treatment of malaria, schistosomiasis and other infectious diseases.
Proteases are involved in many important biological processes including protein turnover, blood coagulation, complement activation, hormone processing, and cancer cell invasion. Thus, they are frequently chosen as targets for drug design and discovery. The critical role certain proteases play in the life cycle of parasitic organisms also makes them attractive drug design targets for certain infectious diseases.
Schistosomiasis (bilharziasis) is a parasitic disease caused by schistosomes (blood flukes) that generally live in the veins of the gut and liver of a human host. Adult worms can survive up to 20 years. Female adult worms release thousands of eggs each day, which often find their way to tissues such as liver, brain, and lung, where they cause considerable damage by stimulating the body to form inflammation and scar tissue around them. Most eggs pass through the bladder or wall of the gut. Once outside, they hatch and infect water snails. The parasite multiplies inside the snail, giving rise to thousands of cercariae that exit the snail and swim free in search of a host in which to complete their life cycle.
Malaria is another well known infectious disease caused by protozoa of the genus Plasmodium, which are transmitted by bites of infected mosquitoes. Infection with Plasmodium falciparum, the most virulent human malarial pathogen, is estimated to be responsible for over 1 million deaths each year. The most valuable of the heretofore developed classes of antimalarial drugs are the quinoline-containing compounds, such as chloroquine and mefloquine; chloroquine has been especially effective as both a preventative and a curative. A serious problem in the treatment and control of malaria has been the increasing resistance of populations of P. falciparum to these known antimalarial drugs. In addition, reports of multi-drug resistance makes the search for novel therapies especially urgent. Thus, there remains a great need to identify new compounds that have antimalarial capabilities.
During the trophozoite stage, the parasites infect red blood cells (erythrocytes) where they reproduce asexually. At the completion of each asexual cycle, the red blood cells lyse and merozoites are released which invade new red blood cells. This cycle of lysis and re-infection is responsible for the major clinical manifestations of malaria.
Most anti-malarials are blood schizontocides which are active against the parasites during the intra-erythrocytic stage of its life cycle. Sulphones and sulphonamides inhibit the synthesis of dihydrofolic acid, while biguanides and diaminopyrimidines inhibit the synthesis of tetrahydrofolic acid. Although the mechanism of these anti-malarials is known to involve interference with the parasites' ability to synthesize nucleic acids [Bruce-Chwatt, L. J., Essential Malariology (Wiley, New York (1985))], the mechanism of action of the quinoline-containing compounds has until recently been surprisingly elusive. Recent work provides strong evidence that the quinoline derivatives work by interfering with the detoxification activity of a heme polymerase [Slater and Cerami, Nature 355, 167 (1992)].
During the erythrocytic phase, the parasites degrade hemoglobin as a primary cysteine protease involved in the degradation of hemoglobin, the parasites' primary source of amino acids [Rosenthal, P. J. et al., J. Clin. Invest. 82, 1560 (1988)]. Blocking this enzyrme with cysteine protease inhibitors (such as E-64 and Z-Phe-Arg-FMK) in culture arrests further growth and development of the parasites [Rosenthal, P. J. et al., Mol. Biochem. Parasitol. 35, 177 (1989)]. Because humans (and, probably, most other mammals) do not have an analogous hemoglobinase, inhibition of this protease (either alone or in conjunction with established therapies) provides an attractive strategy for the treatment of malaria. Moreover, inhibition of analogous proteases present in other metazoan parasites would similarly provide potentially valuable techniques for treatment of human and animal patients infected with those parasites.
It is an object of the present invention to provide compositions and methods for treatment of malaria.
It is a further object of the present invention to provide compositions and methods useful in the treatment of other infectious diseases caused by metazoan parasites.